[Technical Field]
5 [0001]
The present invention relates to a steel sheet.
Priority is claimed on Japanese Patent Application No. 2020-049120, filed in
Japan on March 19, 2020, the content of which is incorporated herein by reference.
[Background Art]
10 [0002]
In recent years, efforts have been being made to reduce carbon dioxide emission
in a number of fields from the viewpoint of the global environment protection.
Automobile manufacturers are also actively developing techniques for weight reduction
in vehicle bodies for the purpose of fuel consumption reduction. A decrease in the
15 weight of steel to be used, such as a decrease in the sheet thickness of a steel sheet,
makes it possible to easily decrease the weight of vehicle bodies. However, in the case
of automobiles, improvement in the impact resistance is also emphasized in order to
ensure passenger safety, and thus weight reduction in vehicle bodies by a decrease in the
weight of steel to be used or the like, which is easy, cannot be adopted, and weight
20 reduction in vehicle bodies is not easy. Accordingly, studies are underway to thin
members using high strength steel sheets in order to satisfy both weight reduction in
vehicle bodies and collision resistance. Incidentally, steel sheets to be applied to vehicle
components are formed into component shapes, and, normally, the formability
deteriorates as the strengths of the steel sheets increase. Therefore, there is a strong
25 desire for steel sheets to be applied to vehicle components to have both a high strength
1
and excellent formability. Specifically, for steel sheets that are used for inner sheet
members, structural members, suspension members, and the like of automobiles, stretch
flanging (hole expansion) or bending is often used, and thus the steel sheets need to have
a high strength and to be excellent in terms of elongation, stretch flangeability and
5 bending workability.
[0003]
For example, as described in Patent Document 1, as a steel sheet from which
excellent elongation can be obtained, a dual-phase steel sheet (hereinafter, DP steel)
composed of a composite structure of soft ferrite and hard martensite is known.
10 However, the DP steel sheet is excellent in terms of elongation, but cracks occur in some
cases due to the formation of voids in the interface between ferrite and martensite, which
have significantly different hardness, and thus there is a case where the DP steel sheet is
poor in terms of stretch flangeability or bending workability.
15
[0004]
In addition, Patent Document 2 proposes a high strength hot-rolled steel sheet
that is obtained by setting the cooling rate in a temperature range from the solidification
of a slab to 1300°C to 10 to 300 °C/min and, after finish rolling, coiling the slab at 500°C
or higher and 700°C or lower and has a steel structure composed of a ferrite single phase
and a tensile strength of 1180 MPa or more. Patent Document 2 di scloses that the high
20 strength hot -rolled steel sheet is excellent in terms of the bending workability. However,
the high strength hot-rolled steel sheet described in Patent Document 2 is manufactured
by reheating a slab without cooling the slab to lower than 900°C where ferrite begins to
be formed and hot-rolling the slab. Therefore, there is a problem in that segregation
formed during solidification is not sufficiently reduced and there is a case where the
25 bending workability is not stable. In addition, in Patent Document 2, the stretch
2
flangeability is not taken into account.
[0005]
Patent Document 3 proposes a method for manufacturing a steel sheet having a
ferrite area fraction of 80% or more and a tensile strength of 980 MPa or more by
5 completing hot rolling within five hours after continuous casting to form a solid solution
of Ti exceeding the solubility in y and precipitating fine TiC together with ferritic
transformation during coiling at 550°C or higher and 700°C or lower and a high strength
hot-rolled steel sheet that is obtained by the manufacturing method. However, even in
Patent Document 3, since continuous casting through the completion of hot finish rolling
10 is performed in an austenite region to suppress the precipitation of coarse TiC, there has
been a case where the bending workability deteriorates due to Mn segregation. In
addition, in Patent Document 3 as well, similar to Patent Document 2, the stretch
flangeability is not taken into account.
[Citation List]
15 [Patent Documents]
[0006]
20
[Patent Document 1]
Japanese Unexamined Patent Application, First Publication No. H6-128688
[Patent Document 2]
Japanese Unexamined Patent Application, First Publication No. 2014-194053
[Patent Document 3]
Japanese Unexamined Patent Application, First Publication No. 2014-208876
[Summary of the invention]
[Problems to be Solved by the Invention]
25 [0007]
3
The present in venti o n h as b een made in con sideration of the above-d escribed
proble ms, and an object of the present inventi o n i s to provide a st eel sheet h av in g a hi gh
strength and being excelle nt in t erms of elon g ation , stretch flan geability a nd bending
workability. Here, the steel sheet of the p resent invention al so includes steel sheets
5 hav ing a cov er such as a plating layer on the surface.
[Means for Solving the Problem]
[0008]
The present in ventor s studie d steel sh eet s that are favorable in all of the strength,
the el o n gation, the stretc h flan geability a nd the be nding worka bilit y. A s a r es ult, it wa s
10 found that a steel sheet having a high strength and being excellent in terms o f elo ngation ,
stretch flangeability and bending work ability can be manufactured b y optimiz in g the
che mical co mpo s ition and manufacturing conditions to control the mi crostm ctur e of the
steel sheet and Mn segregat io n and contr o lling the precipitation form of a Ti- b ased
carbid e.
15 [0 009]
The present inventi on h as been ma de based on the a b ove-describ ed finding , and
the gi st o f the present inventi on is as described below.
[0010]
[1] A steel shee t according to a n aspect of the present inve nti o n contains, as a
20 ch emical compo sition, by mass%, C: 0.050% to 0 .250%, Si: 0.005% to 2.000%, M n:
0. 10% t o 3. 00%, P: 0.100% or less , S: 0. 01 00% or less, sol. Al: 0 . 001% to 1.00%, Ti:
0 .150 % t o 0 .400%, N : 0 .0 010% to 0.0 100%, N b: 0% to 0 . 100%, V: 0% to 1. 000%, Mo :
0% to 1.000% , Cu: 0% to 1.00%, N i : 0 % t o 1.0 0%, Cr: 0 % t o 2.00%, W: 0 % t o 1.000%,
B : 0% t o 0.0020%, Ca: 0 % t o 0.0100%, M g: 0 % to 0.0100 %, REM: 0% to 0.0 100% a nd
25 B i: 0% to 0 .0200% with a rem a ind er of Fe and impurities, in whi ch Ex. C obtained b y
4
the following formula (1) is 0.020% or less, a microstructure at a 1/4 depth position of a
sheet thickness from a surface contains 60% or more of ferrite, 0% to 5% ofMA and a
total of 0% to 5% of pearlite and cementite with a remainder of bainite in terms of area
fractions , in the microstructure, the average crystal grain diameter is 10.0 J.lm or less, the
5 average aspect ratio of crystal grains is 0.30 or more, the standard deviation of a Mn
concentration is 0.60 mass% or less, a Ti-based carbide having a Baker-Nutting
orientation relationship in the ferrite is precipitated in a semi-coherent state, and a tensile
strength is 980 MPa or more.
10
15
Ex. C =(%C)- 12{ (%Ti*)/48 + (%V)/51 + (%Nb)/93 + (%Mo)/96 + (%W)/184}
Formula (1)
Here, "%Ti*" in the formula (1) is obtained from the following formula (2).
%Ti* = %Ti- 48 x { (%N)/14 + (%S)/32) Formula (2)
%C, %V, %Nb, %Mo, %W, %Ti, %N and %Sin the formula (1 ) and the formula
(2) are the amounts of C, V, Nb, Mo, W, Ti, Nand S in the steel sheet by mass%.
[2] The steel sheet according to [1] may contain, as the chemical composition,
by mass%, one or more selected from the group consisting of Nb: 0.001% to 0. 100%, V:
0.005% to 1.000%, Mo: 0.001% to 1.000%, Cu: 0.02% to 1.00%, Ni: 0.02% to 1.00%,
Cr: 0.02% to 2.00%, W: 0.02% to 1.000%, B: 0.0001% to 0.0020%, Ca: 0.0002% to
0.0100%, Mg: 0.0002% to 0.0100%, REM: 0.0002% to 0.0100%, and Bi: 0.0001% to
20 0.0200%.
25
[3] The steel sheet according to [1] or [2], in which a plating layer may be
formed on a surface.
[4] The steel sheet according to [3], in which the plating layer may be a hot-dip
galvanized layer.
[5] The steel sheet according to [4], in which the hot-dip galvanized layer may
5
be a hot-dip galvannealed layer.
[Effects of the Invention]
[0011]
According to the above-described aspect of the present invention, it is possible
5 to provide a steel sheet having a high strength and being excellent in terms of elongation,
stretch flangeability and bending workability. The steel sheet of the present invention is
preferable as a material that is used in uses for automobiles, home appliances, mechanical
structures, construction and the like, and, in particular, when the steel sheet is used as a
material for components such as inner sheet members, structural members, suspension
10 members, and the like of automobiles, not only is a contribution made to weight
reduction in vehicle bodies and improvement in impact resistance but the steel sheet is
also easily worked into component shapes.
15
[Embodiments of the Invention]
[0012]
Hereinafter, a steel sheet according to an embodiment of the present invention
(the steel sheet according to the present embodiment) will be described below in detaiL
However, the present invention is not limited only to the configuration disclosed in the
present embodiment and can be modified in a variety of manners within the scope of the
gi st of the present invention.
20 [0013]
First, the chemical composition of the steel sheet according to the present
embodiment will be described.
Numerical value limiting ranges expressed below using "to" include the values
at both ends as the lower limit and the upper limit in the ranges. However, numerical
25 values expres sed with 'less than' or 'more than' are not included in numerical value
6
ranges. In the following description, "%"regarding the chemical composition of the
steel sheet indicates "mass%" in all cases.
[0014]

5 (C: 0.050% to 0.250%)
10
C is an element that bonds to Ti or the like to form a carbide, thereby increasing
the tensile strength of steel. When the C content is less than 0.050%, it becomes
difficult to obtain a tensile strength of 980 MPa or more. Therefore, the C content is set
to 0.050% or more. The C content is preferably set to 0.070% or more.
On the other hand, when the C content is more than 0.250%, there is a concern
about a deterioration of the weldability. Therefore, the C content is set to 0.250% or
less. The C content is preferably 0.220% or less, more preferably 0.200% or less and
still more preferably 0.180% or less.
[0015]
15 (Si: 0.005% to 2.000%)
Si is an element having an action of increasing the tensile strength of steel by
solid solution strengthening and the enhancement of hardenability. In addition, Si is an
element that also has an action of suppressing the precipitation of cementite. When the
Si content is less than 0.005%, it becomes unlikely for the above-described action to be
20 exhibited. Therefore, the Si content is set to 0.005% or more. The Si content is
preferably 0.010% or more.
On the other hand, when the Si content is more than 2.000%, the surface
properties of the steel sheet significantly deteriorate due to surface oxidation in a hot
rolling step. Therefore, the Si content is set to 2.000% or les s. The Si content is
25 preferably 1.500% or less and more preferably 1.300% or less.
7
[0016]
(Mn: 0.10% to 3.00%)
Mn is an element having an action of increasing the tensile strength of steel by
solid solution strengthening and the enhancement of hardenability. When the Mn
5 content is less than 0.10%, ferritic transformation is excessively promoted, and a
Ti-based carbide is coarsely precipitated together with the ferritic transformation at high
temperatures. In this case, it becomes difficult to obtain a tensile strength of the steel
sheet of 980 MPa or more. Therefore, the Mn content is set to 0.10% or more. The
Mn content is preferably 0.30% or more and more preferably 0.50% or more.
10 On the other hand, when the Mn content is more than 3.00%, ferritic
transformation and bainitic transformation are delayed, and a desired ferrite area fraction
cannot be obtained. In this case, the elongation deteriorates, and the formation of MA
degrades the stretch flangeability or the bending workability. Therefore, the Mn content
is set to 3.00% or less. The Mn content is preferably 2.50% or les s, more preferably
15 2.00% or less and still more preferably 1.50% or less.
[0017]
(soL Al: 0.001% to 1.00%)
Al is an element having an action of cleaning steel by deoxidation in a
steelmaking stage. When the soL Al content is less than 0.001%, it becomes difficult to
20 exhibit the above-described action. Therefore, the soL Al content is set to 0.001% or
more. The soL Al content is preferably 0.01% or more, more preferably 0.02% or more
and still more preferably 0.03% or more.
On the other hand, even when the sol. Al content is set to more than 1.00%, the
effect of the above-described action is saturated, and the refining cost increases.
25 Therefore, the sol. Al content is set to 1.00% or less. The soL Al content is preferably
8
0.80% or less and more preferably 0.60% or less. sol. Al refers to acid-soluble AL
[0018]
(Ti: 0.150% to 0.400%)
Ti is an element that bonds to C to form a Ti-based carbide and contributes to
5 increase in the tensile strength of the steel sheet. In addition, Ti is an element having an
action of refining the microstructure by forming a Ti nitride to suppress the coarsening of
austenite during the reheating and hot rolling of a slab. When the Ti content is less than
0.150%, it becomes difficult to obtain a tensile strength of 980 MPa or more due to the
lack of the precipitation hardening amount. Therefore, the Ti content is set to 0.150%
10 or more. The Ti content is preferably 0.170% or more, more preferably 0.190% or more
and still more preferably 0.210% or more.
On the other hand, when the Ti content becomes excessive, a coarse Ti-based
carbide remains in austenite in an undissolved state, which degrades the elongation or the
bending workability, and the amount of a Ti-based carbide having a Baker-Nutting
15 orientation relationship contributing to the strength, which decreases the strength.
20
25
Therefore, the Ti content is set to 0.400% or less. The Ti content is preferably 0.380%
or less and more preferably 0.350% or less.
[0019]
(N: 0.0010% to 0.0100%)
N is an element having an action of refining the microstructure by forming a Ti
nitride to suppress the coarsening of austenite during the reheating and hot rolling of a
slab. When the N content is less than 0.0010%, it becomes difficult to exhibit the
above-described action. Therefore, the N content is set to 0.0010% or more. The N
content is preferably 0.0015% or more and more preferably 0.0020% or more.
On the other hand, when the N content is more than 0.0100%, a coarse Ti nitride
9
is formed, and the stretch flangeability of the steel sheet deteriorates. Therefore, theN
content is set to 0.0100% or less. TheN content is preferably 0.0060% or less and more
preferably 0.0050% or less.
[0020]
5 (P: 0.100% or less)
Pis an element that is contained in steel as an impurity and has an action of
degrading the stretch flangeability or bending workability of the steel sheet. Therefore,
the P content is set to 0.100% or less. The P content is preferably 0.060% or less, more
preferably 0.040% or less and still more preferably 0.020% or less. Pis mixed from a
10 raw material as an impurity, and the lower limit thereof is not particularly limited, but the
P content is preferably as small as possible from the viewpoint of ensuring the bending
workability. However, when the P content is excessively decreased, the manufacturing
cost increases. From the viewpoint of the manufacturing cost, the P content is
preferably 0.001% or more and more preferably 0.005% or more.
15 [0021]
(S: 0.0100% orless)
S is an element that is contained in steel as an impurity and has an action of
degrading the stretch flangeability or bending workability of the steel sheet. Therefore,
the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less,
20 more preferably 0.0060% or less and still more preferably 0.0030% or less. S is mixed
from the raw material as an impurity, and the lower limit thereof is not particularly
limited, but the S content is preferably as small as possible from the viewpoint of
ensuring the bending workability. However, when the S content is excessively
decreased, the manufacturing cost increases. From the viewpoint of the manufacturing
25 cost, the S content is preferably 0.0001% or more, more preferably 0.0005% or more and
10
s till m ore preferably 0.0010% or more.
[0022)
The remainder of the chemical composition of the steel sh eet according to the
present embodiment includes Fe and impurities. In the present embodiment, the
5 impurity means a substance that i s mixed from ore as a raw material, a scrap, the
manufacturing environment or the like and is allowed to an extent that the steel sh eet
according to the present embodiment is not adversely affected.
The steel sheet according t o the present embodiment may contain the following
optional elements instead of some of Fe. Since the steel sheet according to the pre sent
10 embodiment is capable of solving the problems even when the optional e le ments are not
contained, the lower limit of the amount of the optional elements i s 0%.
[0023]
(Nb : 0% to 0.100%)
N b i s an optional elem ent. Nb i s an e le ment hav ing effects on the suppress ion
15 of th e coarsening of the crystal grain diameters o f the steel sheet a nd an increase in the
tensile strength of the steel sheet by th e refinement of the ferrite grain diameters or
precipitation hardening attributed to the precipitation of Nb as NbC. In order to obtain
these effects, the Nb content i s preferabl y set to 0.001% or more. The Nb co ntent is
more preferably 0.005 % or more and still more prefe rably 0.010% or more.
20 On the other hand, w h en the N b cont ent exceeds 0.100%, the above-described
effects are saturated , and there is a concern about an increase in the rolling forc e during
fini sh rolling. Therefore, in a case w here N b is contained, the Nb content is set to
0 . 100% or le ss. The Nb content i s preferably 0.060% or less and more preferably
0.030% or less.
25 [0024]
11
(V: 0 % to 1.000%)
Vis an optional eleme nt. Vis a n el ement having effect s on an increase in the
te ns ile stre n gth of the steel sh eet by the formatio n of a solid solutio n in st eel and inc rease
in the te n sile strength of the steel sh eet by precipitation hardening attributed to the
5 pre cip itation of V a s a carbide , a nitride, a carb o nitride or the like in steel. In orde r to
o btain these e ffects, the V co nt ent is pre fer abl y set to 0.005% o r m ore. The V content i s
m ore pre fe r ably 0.010% or m o r e and still m ore pre fe rably 0 .050 % or m or e.
On the other h a nd, wh en the V conte nt exceeds 1.000%, a c arbide is like ly to
become coarse and there is a case where th e bending workability d eterio rates.
10 Therefore , in a case where Vis containe d , the V content is set to 1.000% or less. Th e V
c o ntent i s preferably 0.800% or less and more preferably 0.600% or l ess.
[00 25]
(Mo: 0% t o 1 .000%)
Mo is an optional e le m ent. Mo i san e lem e nt hav ing effects o n the
15 high-s tre n g th ening of the steel sh eet by the e nhancement of the hard e nability o f steel and
the formatio n of a carbide or a carbo nitride. In order to obtain these effects, the Mo
conte nt i s preferably set to 0.001 o/o or m ore. The Mo content is more prefer ably 0.005 %
o r m ore, still more preferably 0 .01 0% or more and far still more pre fe r a bly 0 .05 0 % or
20
m ore.
On the other hand, wh en the M o content exceed s 1.000%, ther e i s a case wh er e
th e cr acking sensitivity o f a st eel m a terial su ch as a slab i s enhanced . T h er efore, in a
case whe re M o is containe d , the Mo content i s set to 1.000% or less. The Mo conte nt i s
mo r e pr efer ably 0.800% or less a nd still m o re prefe rably 0.600% or l ess.
[0026]
25 (Cu : 0 % to 1.00%)
12
Cu is an optional element. Cu is an element having an effect on improvement
in the toughness of steel and an effect on an increase in the tensile strength. In order to
obtain these effects, the Cu content is preferably set to 0.02% or more.
On the other hand, when Cu is excessively contained, there is a case where the
5 weldability of the steel sheet deteriorates. Therefore, in a case where Cu is contained,
the Cu content is set to 1.00% or less. The Cu content is preferably 0.50% or less and
more preferably 0.30% or less.
10
[0027]
(Ni: 0% to 1.00%)
Ni is an optional element. Ni is an element having an effect on improvement in
the toughness of steel and an effect on an increase in the tensile strength. In order to
obtain these effects, the Ni content is preferably set to 0.02% or more.
On the other hand, when Ni is excessively contained, the alloying cost increases,
and there is a case where the toughness of the steel sheet in a welded heat -affected zone
15 deteriorates. Therefore, in a case where Ni is contained, the Ni content is set to 1.00%
or less. The Ni content is preferably 0.50% or less and more preferably 0.30% or less.
[0028]
(Cr: 0% to 2.00%)
Cr is an optional element. Cr is an element having an effect on an increase in
20 the tensile strength by the enhancement of the hardenability of steeL In order to obtain
this effect, the Cr content is preferably set to 0.02% or more. The Cr content is more
preferably 0.05% or more and still more preferably 0.10% or more.
On the other hand, when the Cr content become excessive, the chemical
convertibility deteriorates. Therefore, in a case where Cr is contained, the Cr content is
25 set to 2.00% or less. The Cr content is preferably 1.50% or less, more preferably 1.00%
13
or less and still more preferably 0.50% or less.
[0029]
(W: 0% to 1.000%)
W is an optional element. W is an element having an effect on an increase in
5 the tensile strength by the formation of a carbide or a carbonitride. In order to obtain
this effect, theW content is preferably set to 0.020% or more.
On the other hand, even when more than a certain amount ofW is contained, the
effect of the above-described action is saturated, and thus the alloying cost increases.
Therefore, in a case where W is contained, theW content is set to 1.000% or less. The
10 W content is preferably 0.800% or less.
[0030]
(B: 0% to 0.0020%)
B is an optional element. B is an element having an effect on an increase in the
tensile strength of the steel sheet by grain boundary strengthening or solid solution
15 strengthening. In order to obtain this effect, the B content is preferably set to 0.0001%
or more. The B content is more preferably 0.0002% or more.
On the other hand, even when more than 0.0020% of B is contained, not only is
the above-described effect saturated, but the alloying cost also increases. Therefore, in
a case where B is contained, the B content is set to 0.0020% or less. The B content is
20 more preferably 0.0015% or less.
[0031]
(Ca: 0% to 0.0100%)
Ca is an optional element. Ca is an element having an effect on the refinement
of the microstructure of the steel sheet by the dispersion of a number of fine oxides in
25 molten steel. In addition, Ca is an element having an effect on improvement in the
14
5
stretch flangeability of the steel sheet by fixingS in molten steel as spherical CaS to
suppress the formation of an elongated inclusion such as MnS. In order to obtain these
effects, theCa content is preferably set to 0.0002% or more. TheCa content is more
preferably 0.0005% or more and still more preferably 0.0010% or more.
On the other hand, when theCa content exceeds 0.0100%, the amount of CaO in
steel increases, and there is a case where the toughness of the steel sheet deteriorates.
Therefore, in a case where Ca is contained, theCa content is set to 0.0100% or less.
TheCa content is preferably 0.0050% or less and more preferably 0.0030% or less.
[0032]
10 (Mg: 0% to 0.0100%)
Mg is an optional element. Similar to Ca, Mg is an element having effects on
the suppression of the formation of coarse MnS by the formation of an oxide or a sulfide
in molten steel and the refinement of the microstructure of the steel sheet by the
dispersion of a number of fine oxides. In order to obtain these effects, the Mg content is
15 preferably set to 0.0002% or more. The Mg content is more preferably 0.0005% or
more and still more preferably 0.0010% or more.
On the other hand, when the Mg content exceeds 0.0100%, an oxide in steel
increases, and there is a case where the toughness of the steel sheet deteriorates.
Therefore, in a case where Mg is contained, the Mg content is set to 0.0100% or less.
20 The Mg content is preferably 0.0050% or less and more preferably 0.0030% or less.
[0033]
(REM: 0% to 0.0100%)
REM is an optional element. Similar to Ca, REM is also an element having
effects on the suppression of the formation of coarse MnS by the formation of an oxide
25 or a sulfide in molten steel and the refinement of the microstructure of the steel sheet by
15
the dispersion of a number of fine oxides. In the case of obtaining these effects, the
REM content is preferably set to 0.0002% or more. The REM content is more
preferably 0.0005% or more and still more preferably 0.0010% or more.
On the other hand, when the REM content exceeds 0.0100%, an oxide in steel
5 increases, and there is a case where the toughness of the steel sheet deteriorates.
Therefore, in a case where REM is contained, the REM content is set to 0.0100% or less.
The REM content is preferably 0.0050% or less and more preferably 0.0030% or less.
Here, REM (rare earth metal) refers to a total of 17 elements including Sc, Y,
and lanthanoids. In the present embodiment, the REM content refers to the total amount
10 of these elements.
[0034]
(Bi: 0% to 0.0200%)
Bi is an optional element. Bi is an element having an effect on improvement in
the formability of the steel sheet by the refinement of the solidification structure. In
15 order to obtain this effect, the Bi content is preferably set to 0.0001% or more. The Bi
content is more preferably 0.0005% or more.
On the other hand, when the Bi content exceeds 0.0200%, the above-described
effect is saturated, and the alloying cost increases. Therefore, in a case where Bi is
contained, the Bi content is set to 0.0200% or less. The Bi content is preferably
20 0.0100% or less and more preferably 0.0070% or less.
[0035]
(Ex. C: 0.020% or less)
Cis precipitated as a Ti-based carbide and contributes to the high-strengthening
of the steel sheet. However, when the amount of C contained is larger than the amount
25 of C to be precipitated as a Ti-based carbide, excess C forms pearlite, cementite, MA or
16
the like and consequently degrades the stretch flangeability or the bending workability.
Ex. C that is obtained by the following formula ( 1) corresponds to the amount of
C contained more than the amount of C to be precipitated as a Ti-based carbide. In the
steel sheet according to the present embodiment, this Ex. Cis set to 0.020% or less. Ex.
5 Cis preferably 0.018 or less and more preferably 0.015% or less. The lower limit is not
10
15
particularly limited.
Ex. C =(%C)- 12{ (%Ti*)l48 + (%V)/51 + (%Nb)/93 + (%Mo)/96 + (%W)/184}
Formula (1)
Here, "%Ti*" in the formula (1) is obtained from the following formula (2).
%Ti* = %Ti- 48 x { (%N)/14 + (%S)/32) Formula (2)
%C, %V, %Nb, %Mo, %W, %Ti, %Nand %Sin the formula (1) and the formula
(2) are the amounts of C, V, Nb, Mo, W, Ti, N and S in the steel sheet by mass%,
respectively.
[0036]
Next, the microstructure of the steel sheet will be described. In the steel sheet
according to the present embodiment, the microstructure at a 114 depth position of the
sheet thickness from the surface contains 60% or more of ferrite, 0% to 5% ofMA and a
total of 0% to 5% of pearlite and cementite with a remainder including bainite. In
addition, in the microstructure, the average crystal grain diameter is 10.0 )lm or less, the
20 average aspect ratio of crystal grains is 0.30 or more, and the standard deviation of the
Mn concentration is 0.60 mass% or less. In addition, a Ti-based carbide having a
Baker-Nutting orientation relationship in the ferrite is precipitated in a semi-coherent
state.
Here, the reason for regulating the microstructure at the 114 depth position of the
25 sheet thickness in the sheet thickness direction from the surface of the steel sheet (a t/4
17
position from the surface in a case where the sheet thickness is represented by t) is that
the microstructure at this position is a typical microstructure of the steel sheet.

[CLAIMS]
What is claimed is :
1. A steel sheet comprising, as a che mical composition, by mass%:
C: 0.050% t o 0.250%;
Si: 0.005% to 2.000%;
Mn: 0.10% to 3.00%;
P: 0.100% or less;
S: 0.0100% or less;
sol. AI: 0.001% to 1.00%;
Ti: 0.150% to 0.400%;
N: 0.0010% to 0.0100%;
Nb: 0% to 0.100%;
V: 0% to 1.000%;
Mo: 0% to 1.000%;
Cu: 0% to 1.00%;
Ni: 0% to 1.00%;
Cr: 0% to 2. 00%;
W: 0% to 1.000%;
B : 0% to 0.0020%;
Ca: 0% to 0.0100%;
Mg: 0% to 0.0100%;
REM: 0% to 0.0100%; and
Bi: 0 % to 0.0200%
with a remainder of Fe and impurities,
wherein E x. C obtaine d by the following formula (I) i s 0.020% or less,
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10
15
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a microstructure at a 114 depth position of a sheet thickness from a surface
contains 60% or more of ferrite, 0% to 5% ofMA and a total of 0% to 5% of pearlite and
cementite with a remainder of bainite in terms of area fractions,
in the microstructure,
an average crystal grain diameter is 10.0 11m or less,
an average aspect ratio of crystal grains is 0.30 or more,
a standard deviation of a Mn concentration is 0.60 mass% or less,
a Ti-based carbide having a Baker-Nutting orientation relationship in the ferrite
is precipitated in a semi-coherent state, and
a tensile strength is 980 MPa or more,
Ex. C =(%C)- 12{ (%Ti*)/48 + (%V)/51 + (%Nb)/93 + (%Mo)/96 + (%W)/184}
Formula (1)
here, "%Ti*" in the formula (1) is obtained from the following formula (2),
%Ti* = %Ti- 48 x { (%N)/14 + (%S)/32) Formula (2)
%C, %V, %Nb, %Mo, %W, %Ti, %Nand %Sin the formula (1) and the formula
(2) are the amounts of C, V, Nb, Mo, W, Ti, Nand S in the steel sheet by mass%.
2. The steel sheet according to claim 1, comprising, as the chemical composition, by
mass%, one or more selected from the group consisting of
Nb: 0.001% to 0.100%;
V: 0.005% to 1.000%;
Mo: 0.001% to 1.000%;
Cu: 0.02% to 1.00%;
Ni: 0.02% to 1.00%;
Cr: 0.02% to 2.00%;
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15
W: 0.02% to 1.000%;
B: 0.0001% to 0.0020%;
Ca: 0.0002% to 0.0100%;
Mg: 0.0002% to 0.0100%;
REM: 0.0002% t o 0.0100%; and
Bi: 0.0001% to 0.0200%.
3. The steel sheet according to claim 1 or 2 ,
wherein a plating layer is formed on a surface.
4 . The steel sheet according to claim 3 ,
wherein the plating layer is a hot-dip galv anized layer.
5. The st eel sheet according to claim 4 ,
wherein the hot-dip galvanized layer i s a ho t-dip gal vannealed layer.